In a mechanical watch movement, the collet forms an assembly interface of the balance spring to the balance. Originally, it was a washer driven onto the balance staff and comprising at least one hooking point of the inner terminal curve of the balance spring, for example by using a conical pin, by bonding or by welding. As welding, in particular laser welding is the preferred method of fixing, the collet can be made of steel, and more particularly special steels incorporating variable proportions of Ni, Mo, Co or Cr. The collet must first of all be of small dimensions so as to have only a slight influence on the moment of inertia, and so as not to introduce any unbalance, but many other properties are expected from such a small part to contribute to the regular working of the regulating member. It is for example necessary, after driving the collet onto the balance staff, to be able to rotate it without any difficulty in order to put the watch into beat, i.e. to align the impulse pin and the line of centres between the balance and the pallet-staff at the position of rest. It is also desirable for the driving in of the collet to have as little as influence as possible on keeping a given distance between the balance staff and the hooking point and that the external contour of the collet is such that it does not disturb the active length of the inner terminal curve of the balance spring.
Numerous Patents filed in the 1960s and 1970s provide a solution to certain quality criteria listed hereinbefore, but no collet possesses all of the qualities required at the same time, as explained briefly hereinafter.
When the collet is made of special steel, the friction torque on the balance staff after driving in can be too great and can make it difficult to put the watch into beat. In order to overcome this drawback, the first solution consists in making a slot of elasticity between the staff hole and the edge of the collet as described for example in CH Patent No. 347 142 for a collet with a perfectly circular contour on which the balance spring is fixed by means of a conical pin located in the plane of symmetry passing through the axes and the slot. CH Patent No. 508 233 discloses a collet of the same type, with a balance spring riveted or bonded in a groove, but of asymmetrical racket shape with the staff hole offset, such that the risk of the first coil pressing on the contour of the collet is avoided. These slotted collets with a circular staff hole have, however, two drawbacks. When driven onto the balance staff, the movement of the hooking point cannot be rigorously controlled, and when the watch is put into beat there is a high risk of disturbing the flat and the centring. In order to reduce the friction torque facilitating putting the watch into beat without having to make a slot, U.S. Pat. No. 3,429,120 proposes making a collet in two parts comprising an inner ring made of brass, or other non-ferrous material, and an external steel part for welding the inner terminal curve of the balance spring. The results obtained are technically satisfactory but the manufacturing and assembling costs are prohibitive.
In order to reduce the friction torque in a special steel collet without a slot of elasticity, it seems a priori logical to reduce the friction surface to discrete contacts between the balance staff and the driving aperture of the collet. The method disclosed in CH Patent No. 311 287 consists in ovalising a hole, which initially has a circular shape, the contour of the collet keeping its initial circular shape. Since there are consequently only two symmetrical bearing points, the same problems as those referred to above for the slotted collet are again experienced.
CH Patent No. 466 807 and U.S. Pat. No. 3,430,435 propose giving the driving aperture of the collet a regular polygonal shape illustrated by an equilateral triangle with broken angles, or an aperture of hypocycloidal shape. In this embodiment it will be observed that the points where the collet is tight on the staff have angular equidistribution and that the collet has almost necessarily a regular contour inscribed in a circle, in particular so as to avoid having problems of unbalance, but having by way of counterpart, the drawback of having a risk of contact of the inner coil of balance spring.